Protective system for series capacitors



Nov. 24, 1953 R. E. MARBURY 2,660,693

PROTECTIVE SYSTEM FOR SERIES CAPACITORS Filed Aug. 18. 1949 Fig.1.

I Figa- Auxiliary Air Tank 32 5 lnsulahon I7 Main Air Tank Compressor WITNESSES: INVENTOR W BY VRTZR? Ralph E. Marbury.

Patented Nov. 24, 1953 UNITED STATES PATENT OFFICE PROTECTIVE SYSTEM FOR SERIES CAPACITORS Application August 18, 1949, Serial No. 110,947

10 Claims. 1

The present invention relates to series capacitor installations in alternating-current lines and, more particularly, to a protective system for series capacitors in high-voltage, synchronous, alternating-current transmission lines where system stability is a primary or limiting consideration in the operation of the line.

Capacitors are connected in series in alternating-current transmission or distribution lines to neutralize part, or all, of the inductive reactance of the line, in order to raise the stability limit, or the power limit, of a transmis sion line, or to improve the voltage regulation of a distribution line. Since such capacitors are connected directly in series in the line and carry the line current, the voltage across the capacitor is proportional to the line current, and in case of a fault on the line, the voltage across the capacitor may rise to many times its normal value. Standard capacitor units, such as are used in series capacitor installations, are capable of withstanding overvoltages of the order of 156% of normal voltage for brief periods, but they cannot be subjected to materially higher overvoltages, even momentarily, without the probability of damage.

It is not practical to utilize capacitors which are capable of withstanding the maximum voltage to which they may be subjected under fault conditions, because of the excessive cost, since the cost of a capacitor increases approximately as the square of the voltage for which it is de signed. A series capacitor installation, therefore, usually con 'sts of capacitor units having a voltage ra g determined on the basis of the normal voltage across the capacitor, together with a protective system for bypassing the capacitor under fault conditions, or other excesscurrent conditions, in order to protect the capacitor against overvoltage. In order to adequately protect the capacitor, the protective system must operate to bypass it substantially instantaneously upon the occurrence of an overvoltage of predetermined magnitude, that is, the

capacitor must be effectively bypassed within the first half-cycle of fault current. Because of this requirement of substantially instantaneous operation, spark gaps are usually used in these protective systems, since no switch, or other device involving moving parts or mechanical mcvemerit, could operate fast enough.

In the protective systems which have usually been used in connection with series capacitors in distribution lines, a spark gap is connected across the capacitor to break down and bypass it immediately upon the occurrence of a predetermined overvoltage, and a switch or contactor is provided for bypassing both the gap and the capacitor immediately after the gap has broken down, in order to extinguish the arc in the gap and to relieve the gap from the excessive heating caused by continued arcing. The switch is opened to interrupt the bypass circuit to restore the capacitor to service after the line current fallen to its normal value, or after the lapse of a predetermined time interval which is made long enough to allow the protective devices of the line to clear the fault. This type of protective system is entirely satisfactory for series capacitors installed in distribution lines, where the primary purpose of the capacitor is to improve the voltage regulation of the line, and the short delay in restoring the capacitor to service after a fault has been cleared is of no particular importance.

When a series capacitor is installed in a highvoltage transmission line, however, where system stability is an important or limiting consideration in the operation of the line, and where the series capacitor is installed for the primary purpose of raising the stability limit, so to increase the amount of power that can be transmitted over the line, the type of protective system which has been successfully utilized in connection with series capacitors in distribution lines is not satisfactory, and the problem of protection is much more difficult. The capacitor must be protected substantially instantaneously upon the occurrence of a predetermined overvoltage, as explained above, and if it is completely bypassed, as in prior protective systems, the capacitor is effectively removed from service during fault conditions when the stability problem is most acute. For this reason, the protective system must operate to restore the capacitor to full effectiveness immediately after the fault has been cleared, so that it will be available to assist in. maintaining stability during the critical transient conditions immediately following clearing of the fault.

In previous attempts to provide a satisfactory protective system for series capacitors in highvoltage transmission lines, the capacitor has been completely bypassed during a fault, and various arrangements have been proposed for interrupting the bypass circuit as rapidly as possible following the return of the line current to, or near, normal value. Even with specially designed fastoperating relays and switches, however, such a system necessarily involves an appreciable delay,

of at least several cycles, after fault has been cleared before the capacitor is restored to service. Thus, with these prior protective systems, the capacitor has been completely removed. from serv ice during the fault and during at least a part of the critical period immediately following clearing of the fault, so that the series capacitor, which is installed primarily for the purpose of improving stability conditions and increasing the amount of power which can be transmitted over the line, is completely removed from service and is not available for its intended purpose during the time when it is most needed. This difficulty in providing adequate protection for series capacitors, without at the same time rendering them ineffective when their presence is most needed, has been the major factor that has retarded or prevented the use of series capacitors in highvoltage transmission lines, although the advantages of series capacitors in such lines have long been recognized.

The principal object of the present invention is to provide a series capacitor installation for high-voltage, synchronous, transmission lines in which the capacitor is protected against overvoltage due to fault currents, or other excesscurrent conditions, but in which the capacitor is fully restored to service immediately after the excess-current condition has passed, that is, within the first half-cycle after the fault is cleared, so that the capacitor is fully effective to assist in maintaining stability during the transient con ditions following the fault.

Another object of the invention is to provide a series capacitor installation for high-voltage transmission lines in which the series capacitor is not completely bypassed during a fault, but is shunted by a resistor which is designed. to limit the voltage across the capacitor to a safe value, so the capacitor is adequately protected. during the fault but is at least partly effective to assist in maintaining stability, and in which the capacitor is restored to full. effectiveness within the first half-cycle after the fault is cleared, or within the first half-cycle in which the voltage across the capacitor fails to rise above a safe value.

A further object of the invention is to provide a protective system for series capacitors in which a spark gap device is connected across the capacitor to provide substantially instantaneous protection against overvoltages, and in which the are in the gap is extinguished, preferably by an air blast, at each current zero during a fault, so that the gap is self-clearing and the arc will not restrike in half-cycle in which the voltage fails to rise to the initial breakdown voltage of the gap, thus restoring the series capacitor to service within the first half-cycle after the fault is cleared.

A further object of the invention is to provide a protective system for series capacitors in which voltage-responsive means, such as a spark gap device, is utilized to connect a resistor in parallel with the capacitor whenever the instantaneous voltage across the capacitor exceeds a predetermined safe value. the resistor being designed to limit the voltage across the capacitor to a safe value, and in which the resistor is disconnected at each current zero so that if the voltage fails to rise above a safe value on the succeeding halfcycle, the resistor remains disconnected and the capacitor remains fully effective. Thus, the capacitor is at least partly effective during a fault, or other excess-current condition, and is restored d to full effectiveness immediately after the fault is cleared.

A still further object of the invention is to provide a high-voltage protective spark gap device, especially adapted for series capacitor protection, which is designed to be made self-clearing by means of an air blast, and which is insulated from ground by an insulator column having means for conveying air from the ground end of the column to the and means for providing an adequate supply of air closely adjacent the gap, so that the air blast can be started as soon as the gap breaks down and maintained ions as the gap continues to are.

The invention will be more fully understood from the following detailed description of an illustrative embodiment, taken in connection with the accompanying drawing, in which:

Figure 1 is a schematic diagram showing a series capacitor installation; and

Fig. 2 is a view, partly in elevation and partly in vertical section, of a protective gap device embodying the invention.

The invention is shown in the drawing einbodied in a series capacitor installation in an alternating-current line I. The line I represents one phase of a high-voltage, three-phase, synchronous transmission line in which a series capacitor is installed for the purpose of raising the stability limit of the line, so as to increase the amount of power that can be transmitted over it. Only one phase has been illustrated in the drawing, for the sake of simplicity, but it will be understood that the other phases include similar apparatus. The series capacitor 2 consists of a suitable number of capacitor units cormected to each other in series or series-parallel to provide the desired capacitive reactance and current capacity, and is connected in series in the line i. The term capacitor, as used herein, is to be understood as including any necessary number of individual capacitor units. Isolating disconnecting switches 3 are preferably provided on each side of the series capacitor 2, and a bypass disconnecting switch 4 is connected across the capacitor, so that the capacitor can be bypassed, and isolated from the line, for inspection and maintenance. The bypass disconnecting switch 4, which is normally open, is shown as being automatically controlled, as hereinafter described, but it will be understood that the switch A may also be controlled manually for bypassing the capacitor whenever desired, and that any necessary additional automatic control means may also be provided.

As previously explained, the series capacitor 2 is subject to dangerous overvoltages in case of a fault, or other excess-current condition, on the line I, and it must be protected substantially instantaneously upon the occurrence of a voltage across the capacitor in excess of a predetermined safe value, such as 150% of the normal voltage. For this purpose, a protective spark gap device 5 is provided, and a resistor 6 is connected in series with the gap device 5. The series-connected gap 5 and resistor 6 are connected across the series capacitor 2, as shown in Fig. 1, so that when the gap 5 breaks down, the resistor G is connected in parallel with the capacitor 2. The gap 5 is designed and adjusted to break down and become conducting whenever the instantaneous voltage across the capacitor exceeds the predetermined safe value. The resistor 5 is de signed so that, when connected in parallel with the capacitor 2, it limits the voltage across the capacitor to a safe value. More specifically, the resistance of the resistor is determined so that the resultant impedance of the parallel combination of resistor and capacitor is such that the voltage drop across the capacitor and resistor, under the maximum fault current that can ocour in the line I, does not exceed the maximum safe voltage to which the capacitor 2 may be subjected. It will be apparent, therefore, that when the resistor 6 is connected in parallel with the capacitor 2, the capacitor is eifectively protected against overvoltage under the worst conditions that can occur, but it is not completely bypassed and removed from service, and some capacitive reactance will still be available in series with the line to assist in maintaining stability during fault conditions.

The spark gap device 5 is a self-clearing gap, so that the arc in the gap is extinguished at each current zero, and does not rcstrike unless the instantaneous voltage in the succeeding halfcycle rises above a predetermined value near the initial breakdown voltage of the gap. Any suitable type of self-clearing gap device may be used, but it is preferred to obtain the self-clearing characteristics by means of a blast of deionizing gas directed through the space between the electrodes of the gap device. In the preferred embodiment of the invention, an air blast is utilized to make the gap 5 self-clearing, and the air blast is controlled by a valve device l, which directs the air through a nozzle 8 into the gap space. The valve device I is automatically controlled, as explained hereinafter, so as to initiate the air blast substantially immediately upon breakdown of the gap 5, that is, as soon as current starts to flow in the circuit of the gap and resistor, and to maintain the air blast until current ceases to flow in the gap circuit, when the air blast is interrupted.

In operation, the gap device 5 breaks down to connect the resistor 5 in parallel with the capacitor 2 whenever the instantaneous voltage across the capacitor 2 exceeds the predetermined safe value for which the gap is set. As soon as the gap 5 has broken down, the air b'last is immediately started. The are in the gap 5, therefore, will be extinguished at each current zero and will not restrike until the voltage across the capacitor reaches the breakdown value on the succeeding half-cycle. Thus, the gap will continue to disconnect the resistor ii at each current zero and to reconnect it during the succeeding half-cycle as soon as the voltage exceeds the predetermined value. This continues until the excess-current condition has passed, and the arc in the gap 5 will then. fall to restrike in the first half-cycle after the fault has been cleared. The air blast is then turned off.

The resistor 6, therefore, functions to limit the voltage across the capacitor 2 to a safe value, providing complete protection for the series capacitor, but without entirely bypassing it and removing it from service, the gap 5 serving to connect the resistor 5 in parallel with the capacitor in. each half-cycle when it is needed, and to disconnect it immediately when it is not needed. In this way, the capacitor is fully effective during the first part of each half-cycle and is partly efifective during the remainder of the half-cycle when it is shunted by the resistor, and it is restored to full effectiveness in the first half-cycle after the fault is cleared. The capacitor is, therefore, at least partly available to assist in maintaining stability during a fault, and is fully availis able during the transient conditions following the fault. It will be apparent that the capacitor is fully protected but is not completely removed from service when it is most needed, as in prior protective systems in which the capacitor was completely bypassed during a fault and during at least part of the critical period following clearing of the fault.

The resistor 6 also serves a further purpose in preventing damage to the capacitor by repeated charging and discharging, which would occur if a self-clearing gap alone were used across the capacitor, since the resistor 6 has a damping effect on the capacitor discharge and thus prevents damage to the capacitor. Damping resistors have been proposed heretofore in connection with spark gaps connected across series capacitors, but such resistors have been of relatively low value intended only for damping the capacitor discharge, and they have not been designed or intended to limit the voltage across the capacitor during fault conditions. The resistor 5 of the present invention, however, is designed primarily to limit the voltage across the protected capacitor and is used in connection with a self -clearing gap which connects it immediately when needed disconnects it substantially instantaneously when it is not needed.

In some instances it may be satisfactory to utilize a smaller, and therefore cheaper, resistor 6. having a lower resistance than described above. so that the capacitor is more or less completely bypassed during the periods when the gap 5 is arcing. When such a resistor is used, however, with the self-clearing gap, the primary advantage of restoring the capacitor to full effectiveness within the first half-cycle after a fault is cleared is retained, so that the capacitor is fully available during the transient conditions immediately following the fault, when it is most needed for maintaining stability. This result is obtained by the use of the self-clearing gap in which the arc is extinguished at each current zero and does not restrike unless the instantaneous voltage again rises above a safe value, so that, in effect, the magnitude of the current observed in each half-cycle and the capacitor is not bypassed unless it is necessary. Thus the time within which the capacitor is fully restored to service after a fault is reduced to an absolute minimum. The resistor 5, even if of relatively low resistance, prevents excessive current transients and limits the inrush current due to df charge of the capacitor, when the gap breaks down, as well as reducing the violence of breakdown in the gap. If a larger resistor, of higher resistance, is used, the additional advantage of only partially bypassing the capacitor during a fault is obtained, as explained above.

The air blast may be supplied to the gap device 5, through the valve l, in any suitable manner. Since the gap 5 must be insulated from ground for the full line voltage, however, and since the line voltage be quite high, of the order of 220 kv., for example, in the type of in." stallations for which this system is primarily 'ntended, there are certain practical problems involved in providing an adequate supply of, air under sufiicient pressure to the 5 to maintain the blast as long as needed. The air blast must be directed into the gap immediately after it has first broken down, without a delay of more than 1 /2 or 2 cycles, and must be maintained long as the gap continues to break down, which may be a considerable number of cycles. These requirements necessitate a substantial supply of air, which must be close to the gap, to avoid the delay involved in getting the air to the gap from a tank at a considerable distance, but the must be insulated for line voltage, which means it is spaced a considerable distance from any equipment at ground potential.

A preferred system for supplying the air blast is illustrated in the drawing. A main compressed air tank 9 is provided and is supplied with compressed air by means of a compressor I driven by a motor II, which may be supplied from any available low-voltage source of control power I2. The motor I I may be controlled automatically in any suitable manner to operate the compressor II] to maintain the air in the main tank 9 at a desired pressure, which may, for example, he the order of 150 pounds per square inch, the compressor being connected to the tank 9 through a check valve I3 in the usual manner. desirable to attempt to insulate the tank 9, the compressor I0 and its driving motor I I from ground, and the tank 9 is therefore connected to an auxiliary air tank I4 by means of an insulating conduit I5, which permits the auxiliary tank I4 to be mounted near the gap 5 and insulated from ground, so that an adequate supply of air can be provided close to the gap. A pressure regulator I6 of any suitable type is preferably interposed in the conduit IE to maintain the pressure in the auxiliary air tank I4 at a desired lower value, which may, for example, be of the order of 100 pounds per square inch, and a check valve I1 is preferably also provided in the conduit it.

The auxiliary air tank I4 is disposed to sup ply the air blast directly to the gap 5 through the valve device I. Any suitable type of valve device I may be utilized which can be opened very rapidly in response to current flow in the circuit of the gap 5 and resistor 6 and closed automatiincludes a second valve member 23 cooperating with a normally open port 24 for exhausting the cylinder 2|. The pilot valve 22 is adapted to be actuated by means of a solenoid 25 and is normally held in its closed position, shown in the drawing, by a tension spring 26, or other suitable biasing means. The solenoid 25 is energized by the current in the circuit of the gap device 5, either directly or by means of a current transformer 21.

When a fault, or other excess-current condition occurs on the line I, the gap 5 breaks down to connect the resistor 6 across the capacitor 2 to limit the voltage across the capacitor, as explained above. As soon as the gap has broken down and current flows through the circuit of the gap 5 and resistor Ii, the solenoid 25 is energized and moves the pilot valve 22 to the right, as viewed in Fig. 1, closing the exhaust port 24 and admitting air from the valve chamber I8 to the cylinder 2|. The air pressure thus applied to the piston 20 moves the main valve I9 to the right to admit air from the valve chamber I8 to the nozzle 8, which directs the air blast into the gap 5. The air blast effectively deionizes the arc space in the gap and thus extinguishes the are It is not at each current zero. As long as the excess-cur"- rent condition still exists, the arc restrikes on each half-cycle as soon as the instantaneous voltage has increased to the predetermined value for which the gap is set, and this continuesv until the fault is cleared, or the current has. returned to its normal value. This continuous restriking of the arc in each half-cycle causes an intermittent current in the circuit of the gap 5 which maintains the solenoid 25 sufficiently energized to keep the pilot valve in its open position, so that the main valve I9 is held open and the air blast is continuous during this period. When the fault is cleared, and the instantaneous voltage across the capacitor fails to rise to the breakdown voltage of the gap, the arc in the gap 5 will fail to restrike, so that current ceases to flow in the circuit of the gap device. The solenoid 25 is thus deenergized and the tension spring 26 returns the pilot valve 22 to the position shown in the drawing, in which the exhaust port 24 is opened and the cylinder 2I closed off from the valve chamber I8. The air pressure in the cylinder 2| is thus exhausted, and the air pressure in the valve chamber I8 acting on the main valve I9 returns it to closed position, interrupting the air blast.

The bypass disconnecting switch 4 is automatically controlled to bypass the capacitor 2 in case of failure of air pressure. The switch 4 is shown diagrammatically as being biased to closed position b a spring 28 and is normally held in the open position by a latch 29. The latch 29 is normally held in position to hold the switch 4 1 open by a solenoid 30, and upon deenergization of the solenoid, the latch 29 is moved by a compression spring 3|, or other suitable biasing means, to release the bypass switch 4 and perrnit it to be closed by the spring 28. The solenoid 30 is normally continuously energized from the control source I2, and the circuit includes the normally closed contacts 32 of a pressure switch 33, which is connected to the auxiliary air tank I4 or the conduit means I5, so as to be responsive to the pressure in the auxiliary tank I4. The

switch 33 is adjusted to open its contacts and permit the latch 23 to release the disconnecting switch 4 to bypass the capacitor 2 if the pressure in the auxiliary tank I4 falls below a predetermined minimum value, which may, for example, be pounds per square inch if the normal pressure in the tank is pounds per square inch.

A preferred practical structure for the gap device 5, and its associated air system, is shown in Fig. 2. As shown in this figure, the main air tank 9, compressor I5, and motor I I are placed in a housing 35, which may be mounted on the ground or on any suitable supporting structure, and which does not have to be insulated from ground. The gap device 5, valve I and auxiliary air tank 54 are insulated from ground for the full line voltage, and are mounted on top of a vertical column of insulating elements 35, which is utilized to convey the air from the main tank 9 to the auxiliary tank I4. The insulators 36 may be hollow porcelain insulators of any suitable type, mounted end-toend in a vertical column, a sufficient number of insulators being used for the particular voltage. The auxiliary air tank I4 is preferably a cylindrical steel tank and is mounted vertically on top of the uppermost insulator 36, as shown. The insulator column may be placed on top of the housing 35, or in any convenient location. The auxiliary air tank I4 is connected to the main air tank 9 by means of an insulating conduit l5, which is preferably made of thick-walled rubber tubing which is capable of withstanding the air pressure, and which can be stressed at a reasonable voltage, which may, for example, be of the order of 2000 volts per foot. The portion or" the conduit which is within the housing 35 may, however, be ordinary pipe and is connected to the rubber tubing at the bottom of the lowermost insulator 36. The rubber tubing extends through the hollow insulators 36, being disposed in a helical coil in each insulator, and the insulators are preferably filled with a suitable insulating compound 3'1. In order to avoid the necessity of providing airtight gaskets or other sealing means between the inin Fig. 2 has massive graphite electrodes which are capable of sustaining continued arcing without excessive heating or other damage. The gap has an upper hollow electrode 49, which is generally cup-shaped with the hollow opening downward, and it has a lower rod-like electrode il,

which is disposed vertically and extends into the hollow of the upper electrode the electrodes being spaced apart to form a gap between them having the desired breakdown voltage. The lower electrode 4! has a central opening 52 extending through it, and a vent pipe 13 is connected to the opening 42 at its lower end, that is. the end remote from the gap. The vent pipe it may extend through the valve housing 39 and discharges into the air, while the nozzle 8 of the Valve device extends upward beside the lower electrode ii, so that the air blast is directed into the space between the electrodes and the air escapes through the opening t2 and the vent pipe '33. The upper electrode Gil is mounted in a metal cap member at, which carries a terminal device of any suitable type for connection of a lead to the upper electrode. The lower electrode ll is mounted on a bottom plate which may also form the top of the valve housing 39, and a suitable bushing 46 is carried on the valve housing 39 for connection of a lead to the electrode ll. The gap device 5 is enclosed in a porcelain housing d? which encloses the gap space and insulates the electrodes from each other.

It will be seen that a high-voltage protective gap device is thus provided which is adequately insulated from ground for the full line voltage and which is made self-clearing by an air blast. The auxiliary air tank M and valve '5 are also insulated from ground and are mounted directly adjacent the gap 5, so that when the valve '1 is opened in response to current flow through the gap, the air blast is initiated immediately without the delay that would be occasioned if the air had to be brought from a main tank at ground potential. The use of the auxiliary tank also makes it possible to provide a relatively large air supply closely adjacent the gap, so that the air blast can be maintained with undiminished intensity during the entire time that the gap 5 continues arcing. Thus, a desirable practical structure is provided for a self-clearing gap device suitable for use in the protective system of Fig. 1.

It should now be apparent that a protective system has been provided for series capacitors in high-voltage, synchronous, alternating-current transmission lines in which the capacitor is adequately protected against overvoltage, but is not completely bypassed, so that it is at least partly effective during a fault, and is restored to full effectiveness immediately after the fault is cleared, so that the capacitor is available to assist in maintaining stability during the time when the stability conditions are most acute, that is, during a fault and the transient conditions immediately following the fault. This result is ob tained by the use of the voltage-limiting resistor and the self-clearing gap which connects the resister in parallel with the capacitor to limit the voltage whenever the instantaneous voltage exceeds a safe value, and which immediately disconnects theresistor when it is no longer necessary.

An illustrative embodiment of the invention, and. of the apparatus utilized in carrying it out, has been shown and described, but it is to be understood that various other embodiments and modifications are possible within the scope of the invention, and, in its broadest aspects, it includes all equivalent embodiments and modifications which come within the scope of the appended claims.

I claim as my invention:

1. A series capacitor installation for an alternating-current line comprising a capacitor adapted to be connected in series in said line, a spark gap device and a resistor connected in series across the capacitor, said spark gap device being adapted to break down and connect the resistor in parallel with the capacitor whenever the instantaneous voltage across the capacitor exceeds a predetermined value, an air tank containing compressed air, means for directing a blast of air from said tank through the gap, valve means for controlling the air blast, means for opening the valve means to start the air blast immediately after the gap has broken down and for closing the valve means to interrupt the air blast when current ceases to flow in the circuit of the gap and resistor, and pressureresponsive means for effecting completion of a bypass circuit around the capacitor if the pressure in the air tank falls below a predetermined value.

2. In combination, a capacitor adapted to be connected in series in an alternating-current line, a spark gap device and a resistor connected in series across the capacitor, said spark gap device including a first electrode having a hollow therein and a generally rod-like second electrode extending into the hollow of the first electrode, said electrodes being spaced apart to form a gap and being enclosed in a housing, the rod-like second electrode having a central opening therethrough, means for directing a blast of gas into the space between the electrodes, vent means connected to the opening in the second electrode adjacent the end thereof remote from the gap,

valve means for controlling the blast of gas, and

means for efiecting opening of the valve means to start the blast immediately upon breakdown of the gap and for effecting closing of the valve means to interrupt the blast when current effectively ceases to flow in the circuit of the gap and resistor.

3. In combination, a capacitor adapted to be connected in series in an alternating-current line, a spark gap device and a resistor connected in series across the capacitor, said spark gap dell vice including a first electrode having a hollow therein and a generally rod-like second electrode extending into the hollow of the first electrode, said electrodes being spaced apart to form a gap and being enclosed in a housing, the rod-like sec: ond electrode having a central opening therethrough, a tank containing compressed air, means for directing a blast of air from said tank into the space between the electrodes, vent means connected to the opening in the second electrode adjacent the end thereof remote from the gap, valve means for controlling the air blast, and current-responsive means for effecting opening of the valve means to start the blast immediately upon breakdown of the gap and for efiecting connected in series in an alternatingscurrent line,

a spark gap device and a resistor connected in series across the capacitor, said spark gap de-. vice including a first. electrode having a hollow therein and a generally rod-like second electrode extending into the hollow of the first electrode, said electrodes being spaced apart to form a gap and being enclosed in a housing, the rod-like sec: and electrode having a central opening there-. through, a tank containing compressed air, means for directing a blast of air from said tank into, the, space between the electrodes, vent means connected to, the opening in the second electrode adjacent the end thereof remote from the gap, valve means, for controlling the air blast, current-responsive. means for eii ecting opening of; the valve means to, start the blastimmediately upon breakdown of the gap, and for effecting closing of the valve means to interrupt the blast when current ceases to, flow in the circuit of the ap. and e i tor, d: pr ur r e v means for effectingcompletion, of a bypass circuit around the. capacitor if the pressure in said tank falls below a predetermined, value.

5, A series capacitor installation for an, alterhat n rr nt n mp sin adap ed; to be, connected in, series in, said line, a spark gap device connected; across said: ca- Dacitor for protecting, the-capacitor against overvoltage conditions, said. spa k gap device hav-v ing spaced solid electrodes of massive construc-. tion. to, be capable. of discharging, heavy currents and being, adapted to, break down and; become conducting. whenever the. instantaneous voltage across the, capacitor exceeds. a, predetermined value, meansfor directing a. blast of: air between the electrodes, of the. spark gapdevice, andameans for starting the. air blast immediately. upon breakdown of the gap, deviceand-g maintaining the air. blast during substantiallythe. entiredura: tion, of; an. overvoltage condition, whereby; the air blast extinguishes thearcinlthegap device each time, the. current: passes. through zero and prevents reestablishment. of: thearc unless the instantaneous voltage across. the capacitorv again rises to substantially said predetermined value.

6. A series capacitorinstallation for analternatingecurrent line, comprising a capacitor adapted to, be connected. in series. in said line, a. spark gap device connected across said: capaci-l torior protecting the capacitor against overvolta age conditions, said spark gap device having mas.- sive graphite-electrodes spacedapart toforman air. gap therebetweenandibeing.adaptedto break down and become conducting Whenever-.the in stantaneous voltage across. the-capacitor exceeds a capacitor a predetermined value, means for directing a blast of air between the electrodes of the spark gap device, and means for starting the air blast immediately upon breakdown of the gap device and maintaining the air blast during substantially the entire duration of an overvoltage cons dition, whereby the air blast extinguishes the arc in the gap device each time the current passes through zero and prevents reestablishment of the arc unless the instantaneous voltage across the capacitor again rises to substantially said predetermined value.

7. A series capacitor installation for an alternating-current line comprising a capacitor adapted to be connected in series in said line, a spark gap device connected across said capacitor for protecting the capacitor against overvoltage conditions, said spark gap device having spaced solid electrodes of massive construction to be capable of discharging heavy currents and bein adapted to break down and become conducting whenever the instantaneous voltage across the capacitor exceeds a predetermined value, means for supplying a, blast of air to the space between the. electrodes of the spark gap. device, valve means for controlling said air blast, and means for actuating said valve means to start the. air blast immediately upon breakdown of the gap device and to maintain the air blast during substantially the entire duration ofan overvoltage condition, whereby the air blast extinguishes the arc in the gap device each time the current passes through zero and prevents reestablishment of the arc unless the instantaneous voltage across the capacitor again rises to substantially said predetermined value.

8. A series capacitor installation for an alterhating-current line comprising a capacitor adapted to be connected in series in said line, a spark gap device connected across said capacitor for protecting thecapacitor against overvoltage conditions, said spark gap device havingmassi-ve graphite electrodes spaced apart to form an air gap therebetween and being adapted to break down and become conducting whenever the instantaneous voltage across the capacitor exceeds a predetermined value, means for supplying a blast of air to thespace between the electrodes of the spark gap device, valve means for controlling said air blastand means for actuating said valve means to start the airblast im mediately upon breakdown of the gap device and to, maintain the air blast during substantially the entire duration of' an overvoltage condition, whereby the air blast extinguishesthe arc the gap device each time the current passes through zero and prevents reestablishment of the are unlessthe instantaneous voltage across the capacitor again, rises to substantially said predetermined value.

9; A series capacitor installation for analternating-current line comprising a capacitor adapted to be connected, in series in said line, a spark gap device connected" across said capacitor for protecting the capacitor against overvoltage conditions, a resistor connected imseries with the spark gap device, saidispark-gap device having. spaced solid; electrodes of, massive construction to be capable of; discharging heavy currents and being adapted to break downand become conducting to connect the resistor across the capacitor whenever the instantaneousvoltage across the capacitor exceeds -a predetermined value, I the resistance of-' saidresistor bein such that the voltageacross the capacitor is limited to a lower predetermined maximum value under the maximum line fault current conditions and that the resultant impedance of the paralleled capacitor and resistor is such that a significant amount of capacitive reactance remains effective in series in the line, means for directing a blast of air between the electrodes of the spark gap device, and means for starting the air blast immediately upon breakdown of the gap device and maintaining the air blast during substantially the entire duration of an overvoltage condition, whereby the air blast extinguishes the arc in the gap device each time the current passes through zero and prevents reestablishment of the arc unless the instantaneous voltage across the capacitor again rises to substantially said first-mentioned predetermined value.

10. A series capacitor installation for an alternating-current line comprising a capacitor adapted to be connected in series in said line, a spark gap device connected across said capacitor for protecting the capacitor against over-voltage conditions, and a resistor connected in series with the spark gap device, said spark gap device having spaced solid electrodes of massive construction to be capable of discharging heavy currents and being adapted to break down and become conducting to connect the resistor across the capacitor whenever the instantaneous voltage across the capacitor exceeds a predetermined value, the resistance of said resistor being such that the voltage across the capacitor is limited to a. lower predetermined maximum value under the maximum line fault current conditions and that the resultant impedance of the paralleled capacitor and resistor is such that a signiiicant amount of capacitive reactance remains efiective in series in the line, means for supplying a blast of air to the space between the electrodes of the spark gap device, valve means for controlling said air blast, and means for actuating said valve means to start the air blast immediately upon breakdown of the gap device and to maintain the air blast during substantially the entire duration of an overvoltage condition, whereby the air blast extinguishes the arc in the gap device each time the current passes through zero and prevents reestablishment of the arc unless the instantaneous voltage across the capacitor again rises to substantially said firstmentioned predetermined value.

RALPH E. MARBURY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,271,794 Stevenson July 9, 1918 1,905,226 Hoard Apr. 25, 1933 2,041,663 Marx May 19, 1936 2,194,145 Hansell Mar. 19, 1940 2,207,577 Buell July 7, 1940 2,345,590 Evans et al. Apr. 4, 1944 2,351,986 Ludwig et al June 20, 1944 2,351,989 Marbury June 20, 1944 2,389,007 Strang et al. Nov. 13, 1945 2,399,367 Marbury Apr. 30, 1946 2,401,009 Marbury May 28, 1946 2,433,756 Haine Dec. 30, 1947 

